[BUGFIX] Fix PHP warning

Classes/Controller/Ajax/UploadController.php

@@ -41,6 +41,9 @@ use TYPO3\CMS\Core\Utility\GeneralUtility;
 class UploadController extends AbstractAjaxController {
 	public const JOB_APPLICATION_TEMP_FOLDER = 'temp';
 
+	/**
+	 * @var mixed|string
+	 */
 	public $jobFolderPath = 'JobApplication';
 
 	/**
@@ -141,7 +144,7 @@ class UploadController extends AbstractAjaxController {
 			$storage = $this->fileAndFolderService->getStorage();
 			$fileName = $storage->sanitizeFileName(
 				strtolower(
-					str_replace(' ', '_', trim($pathInfo['filename'] . '.' . strtolower($pathInfo['extension'])))
+					str_replace(' ', '_', trim($pathInfo['filename'] . '.' . strtolower($pathInfo['extension'] ?? '')))
 				)
 			);
 			// if the job application folder is not existing, create it